Sprinkle const around where appropriate (#1909)

* Sprinkle `const` around where appropriate

- This will make it easier to use `NDS` objects in `const` contexts (e.g. `const` parameters or methods)

* Remove the `const` qualifier on `DSi_DSP::DSPRead16`

- MMIO reads can be non-pure, so this may not be `const` in the future

src/GPU2D_Soft.h

@@ -117,7 +117,7 @@ private:
 
         return rb | g | 0xFF000000;
     }
-    u32 ColorComposite(int i, u32 val1, u32 val2);
+    u32 ColorComposite(int i, u32 val1, u32 val2) const;
 
     template<u32 bgmode> void DrawScanlineBGMode(u32 line);
     void DrawScanlineBGMode6(u32 line);